He said, however, that more parents are now enrolling their children in private faith-based schools.

CRE, a main provider, has about 470 volunteers in the Waikato teaching in 83 schools - down from the 130 schools they taught in five years ago.

"We are not there to evangelise in that sense, we are there to educate about what Christians believe, Christian values and that kind of thing," Salisbury said.

Salisbury said the desire of teachers would be for children to allow Jesus into their hearts.

"In fairness, that would be the view as teachers, and what we would like to see. But when it comes to getting into a school, we are sharing the values and the stories and linking them into the school curriculum," he said

"Seeing the happiness - the joy on children's faces when they come each week, seeing them enjoy the interactions with the stories and the games - that's the highlight of these classes."

The Ministry of Education does not monitor schools, but will investigate a complaint that a school might not be complying with the Education Act.

One Waikato parent who asked to remain anonymous said the part of the act that allows schools to close early was a "loophole" in the law.

His child's school closes a half hour early once a week to accommodate the religious class.

While the school has an opt-out system, he finds it emotionally punishing for his child, who doesn't attend.

"In this time, my child is cleaning the classroom or playing on the iPad," the parent said.

"My child is asking why they can't go the classes where their friends are."

And the issue extends further than Waikato, says Secular Education Network co-leader David Hines.

"For some people, an even bigger complaint is that when they opt their kids out, they get emotional pressure," Hines said.

"The kids get teased for not believing in Jesus. Some of them get bribed to go to these classes, such as getting lollies and presents and stuff and the little ones are especially confused by this.

"One group of parents were even threatened to have their names published in the school newsletter if they publicly opposed the Bible studies classes."

Hines' group has filed a legal challenge against religious instruction in schools with the Human Rights Review Tribunal, claiming children are being bullied over their parents' decision to opt out of Bible lessons.

In February, his group sent through 720 pages of evidence and expects to call 23 witnesses to the tribunal hearing over a number of weeks. The group is awaiting a date.

Hines, a retired lay preacher, said most of the classes are taught to children in their first year or two of school.

"I used to do religious instruction myself when I was a minister many years ago and it's definitely much easier to teach five year olds than it is to teach ten year olds, because they start to get critical."

Hines isn't surprised at the declining numbers.

"There's been a growing awareness and since the census in 2013, which showed a huge number of people were not religious, the climate is more in favour of getting out of it."

Ministry of Education spokeswoman Ellen MacGregor-Reid said it's the board of trustees that decide whether their schools will offer religious instruction.

"If we receive a complaint that a school is not complying with the Education Act, then the ministry would investigate that complaint," MacGregor-Reid said.

"The legislation only applies to state primary and intermediate schools.

"There are a number of providers of religious instruction in New Zealand, but we do not endorse any of the providers or the material they use."